1. Field of the Invention
This invention relates to a fuel filter used in a fuel supply system of a diesel engine, and more particularly to the fuel filter provided with a device for preventing clogging of a filter element with wax crystallized in diesel fuel.
2. Description of the Prior Art
In connection with diesel engines operated on diesel fuel or light oil, wax component contained in the diesel fuel tends to crystallize out at temperatures below about -10.degree. C. and adheres to the filter media of a filter element of a fuel filter thereby to clog the filter media. This interrupts the fuel supply to an engine, resulting in engine power output shortage and engine stalling, which will, accordingly, lower the vehicle speed or even cause the vehicle to stop.
To solve such problems, it has been already proposed to recirculate warmed fuel overflowing from a fuel injection pump back to the fuel filter thereby to warm up the fuel within the fuel filter for the purpose of preventing crystallization of wax component in the fuel. The flow amount of the warmed fuel to be fed to the fuel filter is usually controlled by a regulator valve using a thermally responsive expandable member such as a wax pellet. However, certain drawbacks have been encountered in such a fuel supply system utilizing a regulator valve. The calibration and adjustment of the regulator valve are very difficult because of the unreliability of the thermally responsive expandable member. Moreover, failure in the calibration of the valve may cause an excessive expansion of the thermally expandable member, resulting in the breakdown of the regulator valve.